Abstract P2-03-06: Draft recommendations for human epidermal growth factor receptor 2 (HER2) testing in breast cancer will decrease HER2 positivity rates

Abstract

Abstract Introduction: A focused update of American Society of Clinical Oncology (ASCO) and College of American Pathologist's (CAP) guideline recommendations for HER2 testing in breast cancer (BrCa) was published in May, 2017 for public comment (http://www.cap.org/ShowProperty?nodePath=/UCMCon/Contribution Folders/WebContent/pdf/her2-breast-summary-draft-recommendations-2017.pdf). The focused update for in situ hybridization (ISH) addresses following uncommon scenarios: 1) HER2/CEP17 ratio ≥ 2.0, and HER2 /cell <4.0; 2) HER2/CEP17 ratio <2.0 and HER2 /cell ≥6.0; 3) HER2/CEP17 <2.0 and HER2 /cell ≥4.0 and <6.0. The first two groups are currently reported as positive, the third as equivocal. It is recommended that immunohistochemistry (IHC) performed by the same lab for all these groups. If the IHC testing is 3+ or 0/1+ the final diagnosis will be reported as positive or negative respectively. If the reflex IHC is 2+, a recount of the original ISH area of carcinoma with IHC 2+ staining is recommended. The purpose of this study is to review the results of targeted FISH testing following IHC to predict the effects of proposed guidelines in a high volume national reference lab. Materials and methods: HER2 FISH tests performed on BrCa between 4/2015-5/2017 are included. Our lab offers HER2 testing by IHC (HercepTest™ Dako) and dual probe FISH (Dako IQ). Equivocal (2+) cases showing ≥10% weak or moderate circumferential membrane staining or intense but <10% circumferential staining are circled by a pathologist and reflexed to HER2 FISH with preferential counting performed in the circled areas. Equivocal FISH cases are re-tested with the alternate RAI1 probe (Agilent Technologies). FISH scoring is done manually by 2 people following 2013 ASCO/CAP guidelines with at least 20 cells counted in amplified and non-amplified cases and 40 cells counted in equivocal cases. Results: 2460 HER2 FISH test requests were received during the study period. 7 cases failed at initial testing and 13 cases failed at repeat testing of equivocal cases with reflex probe. 389 (16.2%) cases were amplified, 1686 (68.7%) non amplified, and 369 (15.0%) were equivocal with FDA approved probe set. 116 (32.6%) of equivocal cases re-tested with alternate probe were amplified increasing overall amplification rate to 21%. The table below shows cases that fall under 3 uncommon categories that are proposed for changes in draft guidelines. HER2/CEP17 RatioAverage HER2/CellNumber of Cases with IHC (Total Number)IHC n(result)Results per 2013 ASCO/CAP before Reflex ProbeResults per 2013 ASCO/CAP after Reflex ProbeProposed Guidelines≥ 2< 48 (28)7 (2+)POSITIVE-NEGATIVE 1 (1+) < 2≥ 69 (31)9 (2+)POSITIVE-NEGATIVE < 2≥ 4-698 (372)96 (2+)EQUIVOCAL32/98 (32.7%) POSITIVE1/98 (1%) POSITIVE 1 (3+) 1 (0) Conclusions: In our lab where all FISH tests with prior IHC are counted on targeted areas, all of the cases within the first two groups will be reclassified as negative. The use of reflex probe in equivocal cases is not recommended in the draft recommendations further decreasing positivity rates. Approximately 5% decrease is expected in HER2 FISH positivity rates following proposed recommendations in reference laboratory setting. Citation Format: Johnson E, Gulbahce E. Draft recommendations for human epidermal growth factor receptor 2 (HER2) testing in breast cancer will decrease HER2 positivity rates [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P2-03-06.